1. Field of the Invention
This invention relates to a quasi-phase-matching-type second harmonic generating element (hereafter called SHG) using substrates of non-linear ferroelectric optical substances such as LiTaO.sub.3 (hereafter called LT) and LiNbO.sub.3 (hereafter called LN) and the production method thereof and also of enhancement of the output light and efficiency of the SHG element. 2. Related Art
In recent years, the conversion of semiconductor laser of a wave length of 830 nm into a blue light of half that wave length, 415 nm by use of SHG elements has gained considerable attention as a compact, light weight blue light source.
Especially, those quasi-phase-matching-type SHG elements which feature polarization inversion grids formed with a period of 1-10 micron to match the wave length of the SHG light being generated, are regarded as promising elements for their easy phase matching and high SHG efficiency.
For example, pages 731-732 of the Electronics Letters, 25, 11 (1989), introduce a method as shown in FIG. 2 wherein periodic grids are produced on an LN substrate 21 by Ti diffusion and are heated up to about 1,100.degree. C. to invert selectively the polarization of the periodic grid layers to form triangular polarization inverted areas 31 before producing a light wave guide 13 by means of the proton exchange method therein fundamental wave 14 is injected to obtain SHG light 15.
Also, when using LT substrates, for example, as described on pages 2,732-2734 in the Appl. Phys. Lett. 58 (24) (1991), a method as shown in FIG. 3 is being tested wherein periodic grids are produced on a substrate 11 made of LT by proton exchange rather than Ti diffusion, said periodic grids being heated up to about 600.degree. C. to invert selectively the polarization of the periodic grid layers to produce semi-circular polarization inverted areas 41 before producing a light wave guide 13 by means of the proton exchange method wherein fundamental wave 14 is injected to obtain SHG light 15.
Furthermore, a method to form the polarization inverted areas by means of the electron beam method is also introduced (on pages 828-829 in the ELECTRONICS LETTERS 9th May 1991, Vol. 27, No. 10) wherein it was reported that deep polarization inverted areas were successfully formed to penetrate through an almost l mm thick crystal.
With a quasi-phase-matching-type SHG element, it has been theoretically proved that the efficiency of SHG light generation depends on the cross-sectional shape of the polarization inverted areas and if the cross section is made in a rectangular shape, the efficiency of conversion to a second harmonic can be raised by 4 times or more as compared with that by a triangular shape cross section and, furthermore, the positional accuracy of the polarization converted layers becomes less critical (on pages 1-6 in the Intern. Conf. on Materials for Non-Linear and Electrooptics, Jul. 4-7, 1989).
Nevertheless, with the cross sectional shape of polarization inverted grids formed through the Ti diffusion method using an LN substrate of triangular shape as shown in FIG. 2 as numeral 31 and that of polarization inverted grids formed through the proton exchange method of semi-circular shape as shown in FIG. 3 as numeral 41, SHG light has not yet been generated with an efficiency near that of an SHG element with polarization inverted grids of an ideal rectangular cross-sectional shape.
Also, in Ti diffusion areas, more optical damage tends to occur wherewith the index of refraction changes by strong light and in the proton exchange areas, the non-linear optical coefficient deteriorates making it difficult to exhibit the innate efficiency of SHG generation.
Furthermore, the proton exchange treatment method previously employed includes, as shown in FIG. 9, storing the proton source acid 353 inside a glass container 351 installed in a constant temperature bath 354, a substrate 352 being dipped into said acid 353. In this case, however, the glass container 351 tends to be corroded by the proton source acid and the surface of the substrate tends to become coarse or even cracked by the difference in the crystal orientation, since proton exchange is made in all directions in the substrate, thereby opposing the chemically damaging orientation of the acid. Furthermore, in the conventional method, the formation of polarization inverted grids and production of the light wave guide must be processed separately, thus requiring photo-lithography to be performed two or more times.
With the SHG element as presented in the Applied Physical Letters, the depth of polarization inverted grids is very shallow at 1.6 microns while the width is at 2.1 microns and not particularly suitable as a large output element and heat treatment requisition also is very high at 550.degree. C. The reason for there being a restriction in the depth with such an SHG element is because the diffusion proceeds isotropically and in order to form periodical polarization inverted grids, said diffusion must be confined to a certain extent as otherwise said grids: merge with adjoining grids thus impeding the formation of polarization inverted grids.
With the method employing an electron beam to form polarization inverted areas, since grids are pictured successively by electron beam, the depth and width of the polarization inverted grids in the direction perpendicular to the optical axis intrinsically become uneven, making it difficult to maintain uniformity and to generate SHG light satisfactorily. Also, in this method, it is difficult to form polarization inverted areas of larger size because of the occurrence of electron charge up.